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MODULI(5) OpenBSD Programmer's Manual MODULI(5)
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moduli - Diffie Hellman moduli
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The /etc/moduli file contains prime numbers and generators for use by
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sshd(8) in the Diffie-Hellman Group Exchange key exchange method.
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New moduli may be generated with ssh-keygen(1) using a two-step process.
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An initial candidate generation pass, using ssh-keygen -G, calculates
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numbers that are likely to be useful. A second primality testing pass,
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using ssh-keygen -T provides a high degree of assurance that the numbers
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are prime and are safe for use in Diffie Hellman operations by sshd(8).
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This moduli format is used as the output from each pass.
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The file consists of newline-separated records, one per modulus, contain-
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ing seven space separated fields. These fields are as follows:
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timestamp The time that the modulus was last processed as YYYYM-
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type Decimal number specifying the internal structure of
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the prime modulus. Supported types are:
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2 "Safe" prime; (p-1)/2 is also prime.
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4 Sophie Germain; (p+1)*2 is also prime.
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Moduli candidates initially produced by ssh-keygen(1)
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are Sophie Germain primes (type 4). Futher primality
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testing with ssh-keygen(1) produces safe prime moduli
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(type 2) that are ready for use in sshd(8). Other
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types are not used by OpenSSH.
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tests Decimal number indicating the type of primality tests
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that the number has been subjected to represented as a
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bitmask of the following values:
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0x01 Composite number - not prime.
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0x02 Sieve of Eratosthenes
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0x04 Probabalistic Miller-Rabin primality tests.
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The ssh-keygen(1) moduli candidate generation uses the
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Sieve of Eratosthenes (flag 0x02). Subsequent
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ssh-keygen(1) primality tests are Miller-Rabin tests
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trials Decimal number indicating of primaility trials that
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have been performed on the modulus.
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size Decimal number indicating the size of the prime in
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generator The recommended generator for use with this modulus
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modulus The modulus itself in hexadecimal.
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When performing Diffie Hellman Group Exchange, sshd(8) first estimates
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the size of the modulus required to produce enough Diffie Hellman output
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to sufficiently key the selected symmetric cipher. sshd(8) then randomly
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selects a modulus from /etc/moduli that best meets the size requirement.
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ssh-keygen(1), sshd(8),
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Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
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Protocol, RFC 4419, 2006.
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OpenBSD 4.4 June 26, 2008 2
added
removed
moduli.0
